Castell de Montjuïc is a fortress from 17th century (so says Wikipedia) which was used (apparently) to protect the city from outer attacks. Today it survives as a glamourously lanscaped city park with a wonderful scenery. Just imagine the Barcelonata view from this far up! And if you didn't stumble upon a dungeon or a cannonade every once in 20 steps, you might just as well feel like you've come here to have a picnic or something. Just don't let the prisoners and executions that are said to take place here freak you out.